Android : Call event in AccessibilityService returns null message

I am trying to subscribe to notification events on android using AccessibilityService on Android 2.3.x

   @Override
   public void onAccessibilityEvent(AccessibilityEvent event) 
   {
      if (event.getEventType() == AccessibilityEvent.TYPE_NOTIFICATION_STATE_CHANGED) 
      {
          final String sourcePackageName = (String)event.getPackageName();
          Parcelable parcelable = event.getParcelableData();
          if (parcelable instanceof Notification) 
          {
             List<CharSequence> messages = event.getText();
             if (messages.size() > 0) 
             {
                final String notificationMsg = (String) messages.get(0).toString();                        
                Log.v("NotificationService", "Captured notification message [" + notificationMsg + "] for source [" + sourcePackageName + "]");                   
            }
         }
   }        

But for a incoming call event, the messages.size() == 0. How do we find out the caller ID from accessibility service otherwise.